The present invention relates to a method of detecting the actual location of the system terminal device of a stack type network system by means of the operation of photo couplers and current loops, and automatically installing a terminating resistance to the detected location when necessary. The invention relates also to apparatus for the application of such a method.
In designing a stack type network system, the installation of terminating resistance and the judgment of the location of the system terminal device will be encountered and must be settled. Currently, when in a long distance communication situation, the user shall have to find the terminal end of the cable so that a proper terminating resistance can be installed. If a network system is formed by arranging different systems in a stack, as shown in FIG. 1, a terminating resistance R1 is connected to the input port of the first device 1, the output port of the first device 1 is connected to the input port of a second device 2 by a signal line and a ground line, the output port of the second device 2 is connected to the input port of the third device 3 by a signal line and a ground line, and the output port of the third device 3 is connected to a terminating resistance R3. The terminal resistances R1 and R3 are matched subject to the impedance and characteristics of the system devices connected. When the system devices of the network system are changed, the respective terminal resistances must be relatively changed. For example, if the third system device 3 is taken away, the terminating resistance R3 shall be connected to the output port of the second system device 2. However, because the impedance and characteristics of the second system device 2 are different from that of the third system device 3, an error information or load tends to occur when the terminating resistance R3 is directly connected to the output port of the second system device 2. Besides, the procedure of finding the location of the terminal and replacing the terminating resistance is inconvenient.
Furthermore, in the design of a stack type network system, it may be necessary to detect the installation location of the system terminal device. According to regular methods, the signal lines and the ground lines are respectively connected in series, shown in FIG. 2. However, the system devices of the network system may be damaged because of ground potential differences when the signal lines and the ground lines are respectively connected in series.